The image acquisition of a graphic pattern is typically performed according to two main techniques: the laser scanning technique, wherein the graphic pattern is scanned by a laser beam and the light diffused point by point by the illuminated graphic pattern is gathered on a substantially punctiform sensor and converted into electric signal, and the CCD or CMOS techniques, wherein more points of the graphic pattern are illuminated at the same time (on a total or partial area of the graphic pattern, or on a line) and the light diffused by all the illuminated points is gathered on a one- or two-dimensional optical sensor (of the CCD or CMOS type), capable of converting the light impinging it point by point into electric signals representing the single points of the graphic pattern, simultaneously for all sensitive points. The invention refers to the latter of the two techniques.
Theoretically, the graphic pattern can be illuminated just by the ambient light, but specific illumination means are normally used, typically sets of approximately punctiform illuminating elements (such as light diodes or LED), arranged in a one-dimensional array or in a two-dimensional matrix, according to whether the reading is made by lines or by areas.
The light diffused by the illuminated portion of graphic pattern is gathered by an optical reception system (comprising lenses, diaphragms, mirrors and the like) and focused on the optical sensor. Finally, the optical sensor comprises an array or an ordered matrix of single punctiform sensor elements, each gathering—at the same time as the others—the light coming from the graphic pattern and converting it, always at the same time as the other punctiform elements, into a set of electric signals representing the optical characteristics of the single points of the graphic pattern, thereby electrically reconstructing its image.
The problem of the unevenness of illumination on the area or line to be read is well known in the art. In fact, the central portion of the area or of the line to be read is illuminated more intensely than the peripheral zones. This phenomenon, graphically shown in FIG. 2 in the case of an array of four LEDs, is unavoidably associated to the geometrical arrangement of the single illuminating elements and to the fact that each of them has an emission cone of a certain width. As it can be easily seen in FIG. 2, the central zone receives illuminating power (light energy per area unit) from each of the various LEDs, whereas each of the two rightmost and leftmost zones only receives illuminating power from the closest LED. The resulting distribution curve of the illuminating power has a peak at about the centre and decreases towards the ends. The same thing of course applies in case of two-dimensional illumination.
The result is that the peripheral zones of the graphic pattern are less illuminated than the central zone, and so they diffuse less light, thereby producing an image of the graphic pattern that is distorted from the luminous intensity point of view.
Moreover, the problem of the illumination unevenness is made worse by the uneven transmission of the optical reception system, which normally tends to transmit the illuminating power better in its central zone (close to the optical system axis) than in the peripheral zones. A typical pattern of this phenomenon is shown in FIG. 3, which shows how the power of the light composing the image decreases from the centre towards the edges.
The main effect of the phenomenon described is that the electric signal generated by the optical sensor will depend on the amount of light received, and therefore it will have a variable amplitude pattern in the field of view, according to the distance from the axis of the optical reception system.
The overlapping of this unevenness can create serious problems for the proper acquisition of the image; for example, without corrective measures it may even occur that the noise gathered in the central zone has the amplitude comparable to the signal collected in a peripheral zone. This amplitude unevenness can negatively affect the performance of the equipment for acquiring or reading the graphic pattern, in terms of reduction of the aperture or of the depth of the reading field.
Such effects are further made worse as the reading or acquisition distance increases, since the electric signal becomes weaker.
Several approaches are known in the art to correct this situation.
According to a first approach, the problem is dealt with at the origin, by providing for the central LED to be piloted so as to produce a less intense illumination compared to the peripheral ones. Examples of this approach can be found, for example, in U.S. Pat. Nos. 4,818,847 and 5,144,117.
According to another approach that deals with the problem at the origin as well, the spatial distribution of the LEDs and/or the orientation of their axes are not even; more precisely, the central LEDs are made to be more spaced from one another or their axes are made to diverge towards the peripheral zones. An example of this approach is provided in U.S. Pat. No. 5,354,977.
Another known approach (EP-A-1205871), on the other hand, provides for an intervention during the signal electronic processing; that is, it is accepted that the generated signal is affected by the above unevenness to intervene downstream by a gain system which is variable from zone to zone of the image.